The Fireside Girls are a girls organization, similar to Camp Fire USA and the Girl Scouts. The only known troop in Danville is Troop 46231.

This troop appears to be led (at least partially) by Isabella Garcia-Shapiro, while the organization appears to be led (and founded) by Eliza M. Fireside. It's founding time is unknown, but it can be assumed it is relatively recently due to Eliza still being alive. It is unknown if there are additional troops in Danville, or elsewhere.

The girls show a large variety of skills, and often use these to help Phineas and Ferb with their Big Ideas. They also use their handbook, which contains infomation on a large variety of subjects - such as Time Travel and Car Engines.

Known Rules

Buildings

The Fireside Girls' clubhouse.

The Fireside Girls appear to have two buildings. One of which appears to be in Downtown Danville, and is their official lodge. They also have a clubhouse in the woods, with it's exact location unknown. This clubhouse appears to be secret, with both defensive procedures and a secret knock for access. It appears to have access to basic amenities, with a sink usable. ("Isabella and the Temple of Sap")

Background Information

Every single member of the Fireside Girls have talked (even though some don't have information):

Gretchen was the first Fireside girl to talk in the show after Isabella. This occurred during the episode "The Fast and the Phineas", where only two members of Troop 46231 speak, the first being Isabella. Ariel Winter is listed in the credits as the voice of Gretchen. This made it possible to identify Gretchen as the one with straight light brown hair and wears glasses.

Katie, though her name was never said or shown, interrupts Isabella in the "Out to Launch" episode when Isabella was about to ask Phineas to the dance. She also asks for a pack of Peanuts in De Plane! De Plane!

In the episode "Raging Bully", Milly talks about the brawl at the mall and Holly, who's with Ginger, tells Candace that there are no fliers about the brawl left.

Adyson has a very tiny speaking role in "Tree to Get Ready". When the tree houses return to the yard and the girls hold up scorecards, she yells "Way to Go!" and "Awesome!" She also has a larger speaking role in Isabella and the Temple of Sap, making a sarcastic remark on the "Help Thy Neighbour" patch.

Could the other girls in these pictures be other, unknown Fireside Girls?

Several more girls appear alongside the Fireside Girls in "Jerk De Soleil" and "De Plane! De Plane!" It is unknown if the ones in "Jerk De Soleil" are additional Fireside Girls, but the ones in "De Plane! De Plane!" have been confirmed by Dan that they are not, making it a possibility that these other ones aren't as well.

Katie and Ginger make an early cameo appearance in "Rollercoaster" in a crowd shot during the unveiling of the coaster.

Although it appears the Fireside Girls have identical uniforms, they all have different shirt designs. This is likely due to all Fireside Girl uniforms being homemade. ("Fireside Girl Jamboree")

The Fireside Girls' Emblem

The Fireside Girls have an emblem, which appears on all their buildings and equipment.

Dr. Heinz Doofenshmirtz once made a invention to turn the Fireside Girls invisible because he did not want to have to see them during their fund-raising activities (since they were too cute looking, and their "big pleading eyes" made him feel guilty), but it was not used on them. ("Don't Even Blink")